This invention relates to a very small displacement enlargement mechanism using a piezoelectric element having a piezoelectric effect and a printing head using such a mechanism.
A usual dot type printer has a printing head, which uses a very small displacement enlargement mechanism for transmitting a dimensional strain generated by a piezoelectric effect of a piezoelectric element after enlargement to a drive section to obtain a drive force to drive a printing wire.
FIG. 29 shows such a very small displacement enlargement mechanism in the prior art. In the Figure, reference numeral 1 designates a piezoelectric element for generating a dimensional strain with a piezoelectric effect. The piezoelectric element 1 has a lower stationary section and an upper drive section. The stationary section is secured to a mounting section 3 of a stationary frame 2 made of a metal, and the drive section is secured to a mounting section 4 of the frame 2. The mounting section 4 has two movable coupling sections (a) and (b), via which stems of respective movable members 5 and 6 are coupled to the section 4. The stems of the movable members 5 and 6 are also coupled via movable coupling sections (c) and (d) to the opposite ends of the stationary frame 2. When a dimensional strain is produced in the piezoelectric element 1, the movable members 5 and 6 are rotated about the respective movable coupling sections (c) and (d) and displaced in opposite directions. A bar-like spring member 7 has one end secured to a free end of the movable member 5. A substantially V-shaped spring member 8 has one end secured to a free end of the movable member 6. The other ends of the spring members 7 and 8 are coupled together at a coupling section (g), to which an end of an outwardly extending printing wire 9 is secured. Very small displacement enlargement mechanisms having similar constructions and printing heads of impact type using printing units consisting of such mechanisms are disclosed in U.S. patent specification No. 4,589,786 and Japanese patent disclosures Nos. 59-229349, 59-26273, 60-969, 60-31975 and 60-31976.
With the very small displacement enlargement mechanism having the above construction shown in FIG. 29, a very small dimensional strain is produced in the piezoelectric element 1 in a direction of arrow (A) according to a voltage applied between electrodes of the element 1. This dimensional strain causes rotation of the movable members 5 and 6 about the movable coupling sections (c) and (d) as fulcrum points and also about the coupling sections (a) and (b) as force points in the directions of arrows (B) and (C), so that it is transmitted after enlargement. When the movable members 5 and 6 are displaced, the spring members 7 and 8 receive the respective displacements and are displaced in opposite directions as shown by arrows (D) and (E). Thus, a couple acts on the coupling section (g) in a direction of arrow (F), so that the printing wire 9 secured to the coupling section (g) is driven in a direction of arrow (G).
As is shown, in the prior art very small displacement enlargement mechanism, the dimensional strain of the piezoelectric element 1 is enlarged by the movable members 5 and 6 and spring members 7 and 8 to obtain an enlargement ratio (of several ten times) which is necessary for driving the printing wire 9.
With such a prior art structure, the drive section (x), enlarging section (y) and wire section (z) are arranged substantially on a straight line as shown in FIG. 29, and also the printing wire 9 is driven outwardly. Therefore, the mechanism is inevitably large in size, and a printing wire having a plurality of (for instance, 24) wires, constructed using this mechanism, is considerably large in size and heavy in weight. This is greatly undesired from the standpoint of reducing the size and weight of the apparatus. Such a printing head is heavy in weight and poses problems in the control of its driving.
Further, in order to arrange the wires of the printing head at a predetermined interval, it is necessary to collectively guide the ends of the wires toward a wire-arranging member to let the wire ends be held in an accurate positional relation to one another by 24 holes. Therefore, in a prior art printing head, for instance as shown in Japanese Patent Disclosure No. 59-229349, 12 printing units each including a very small displacement enlargement mechanism and a printing wire are stacked in a sector-like fashion on each side. In this case, it is necessary to stack the very small displacement enlargement mechanisms and spacers alternately and secure the resultant stack by means of a curved screw. For this reason, the assembling of the prior art printing head requires a large number of components and also a large number of assembling steps. Further, it is difficult to obtain a high accuracy assembly.
With the very small displacement enlargement mechanism, when assembling the piezoelectric element, the accuracies of the dimension of the element in the direction of generation of strain and the dimension between the mounting sections, to which the element is secured are important. In order to transmit a displacement of the piezoelectric element which is of the order of several microns to several ten microns at most, there should be no gap between the element and each mounting section in a state, in which the two are assembled together with a very high accuracy. Rather, unless the piezoelectric element is held such that it is slightly compressed, the efficiency of transmission of a displacement of the piezoelectric element to an enlarging mechanism is deteriorated.
However, it is difficult in fact to mount the piezoelectric element in the state noted above by controlling the two dimensional accuracies noted above. If it is possible, the price of the product will be extremely increased. Further, it is very difficult to assemble the two in the above dimensional relation to each other. Usually, the piezoelectric element is loosely inserted into the space between the mounting sections of the very small displacement enlargement mechanism and subsequently the state of mounting of the element is adjusted to hold the element to be compressedly secured.
Prior art techniques for adjusting the state of mounting of the piezoelectric element besides those disclosed in the prior art documents noted above include one, in which one end of the piezoelectric element is secured via a wedge-like spacer member so a securing member (as disclosed in Japanese Utility Model Disclosure No. 59-137041), one, in which the piezoelectric element is secured by providing a mechanical trace in a securing section (as disclosed in Japanese Utility Model Disclosure No. 62-28537), one, in which the piezoelectric element is secured by using a depression formed in a connecting member provided on a base by using a punch or the like (as disclosed in Japanese Utility Model Disclosure No. 62-87839), one, in which the piezoelectric element is secured by forming a base with a hole and raising the base with an oval sectional profile pin inserted in the hole (as disclosed in Japanese Utility Model Disclosure No. 62-87840), and one, in which the piezoelectric element is secured by forming a base with a hole and raising the base with a tapered pin pressure-fitted in the hole (as disclosed in Japanese Utility Model Disclosure No. 62-90141).
In the prior art techniques, however, the piezoelectric element is inserted into the space between mounting sections of the enlarging mechanism, and the state of mounting of the element is adjusted by using a wedge-like spacer member, a mechanical trace, a depression, an oval pin or a tapered pin. This means that a special adjusting operation should be done when mounting the piezoelectric element, so that the assembling operation is not so easy.